Imaging Properties of a Series of Magnetic Electron Lenses

Abstract

The paraxial lens data and the more important first order lens aberrations were determined for a series of symmetrical magnetic lenses of equal lens diameter D but variable air gap width S for a wide range of lens excitations. The range of gap widths covered was S/D=0.2 to S/D=2 The field distribution within the lenses was measured to a high degree of accuracy with a resistance network analogue, and the lens data were then obtained through numerical trajectory tracing. The results are represented as a series of graphs These results are representative of the most common magnetic electron lenses, and should have useful applications in lens design. An approximate formula for the focal length is fapproximately equal to50(DS)1/2Vr/(NI)2. A few design examples are discussed.